In Search of Long Island Rare Plants 3 – Southern Arrowwood

From Steve Young – NY Natural Heritage Program. In New York two varieties of Viburnum dentatum are found, var. lucidum, northern arrowwood, also called Viburnum recognitum in some books, and var. dentatum, southern arrowwood.  Viburnum dentatum var. dentatum is found south of New York. Viburnum dentatum var. lucidum is found throughout the state while Viburnum dentatum var. venosum is found only in Suffolk County in New York and mostly on the very eastern end of Long Island. It is presently considered a rare plant by the New York Natural Heritage Program and ranked as S2 – threatened. In June I surveyed the area around Montauk west to Southampton on the South Fork of Long Island to see how common this shrub really is.  On Eastern Long Island it occurs in maritime shrubland with the more common var. lucidum but it can be distinguished fairly easily by leaf and reproductive characters.  It flowers and fruits about two weeks later than var. lucidum and its leaf petioles and undersides are covered with stellate hairs that are absent on var. lucidum. The photos below show the difference.

Northern and southern arrowwood beside each other. Northern on the left in bloom and southern on the right in bud in early June.

Northern arrowwood is in bloom,

When southern arrowwood is in bud.

Southern arrowwood has stellate petioles and twigs.

Northern arrowwood has glabrous twigs and petioles or with small straight hairs in the petiole groove.

Can you tell which species is which here?

Variety venosum on the top and var. lucidum on the bottom.

The brown rough bark with white lenticels looks similar on both varieties.

Because these plants flower at different times, their leaf characters are different, and they occur together instead of separated geographically, I would tend to call them different species rather than varieties.  Variety venosum has been described as a species,  Viburnum venosum, in the past and I would tend to agree with that taxonomy from what I have seen on Long Island.  I surveyed many roadsides and shrublands on the South Fork in June and southern arrowwood was present in good numbers in most of them. I am now recommending that its rank be lowered from threatened to rare and it put on the Heritage Watch List.  Even though it is more common than we thought it should still be monitored because the non-native viburnum leaf beetle (Pyrrhalta viburni) has been completely defoliating this and a few other viburnum species in parts of New York.

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